Megavoltage computed tomography during radiotherapy

Abstract

Reduced dose megavoltage CT images are obtained using low flux data resulting from leakage through modulating shutters and/or collected by other means and augmented by incomplete high flux data collected during radiation therapy. The ability to construct tomographic projection sets from significantly varying flux rate data is provided by the use of air scans windowed to account for variations in mechanical leaf movement. These methods are also provide a means of imaging the patient entirely during radiation therapy treatments without any additional scan time.

We claim:  
     
       1. A combination radiation therapy and tomographic imaging machine comprising: 
       a megavoltage radiation source directing a radiation beam formed of rays directed generally along an axis;  
       a gantry holding the radiation source to rotate the angle of the axis about a treatment volume;  
       a radiation detector positioned on the gantry across the treatment volume opposite the radiation source to receive and measure radiation of the rays to provide data of a projection;  
       a modulator modulating the flux of the rays passing through the treatment volume;  
       an electronic computer executing a stored program to:  
       (i) control the modulator to direct selected high flux rays at selected axis angles at the patient within the treatment volume according to a radiation treatment plan, the selected rays and angles comprising less than a tomographic projection set, to acquire a high flux incomplete tomographic projection set from the radiation detector;  
       (ii) direct low flux rays at the patient within the treatment volume to acquire low flux data from the radiation detector;  
       (iii) combine the high flux incomplete tomographic projection set and the low flux data to produce an augmented complete tomographic projection set; and  
       (iv) reconstruct a tomographic image of the patient from the augmented complete tomographic projection set.  
     
     
       2. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the low flux data comprises a complete tomographic projection set. 
     
     
       3. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the low flux data comprises an incomplete tomographic projection set. 
     
     
       4. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the modulator includes multiple leaves having open and closed positions to pass or occlude rays and wherein the low flux rays are obtained from leakage through the leaves. 
     
     
       5. The combination radiation therapy and tomographic imaging machine of  claim 4  wherein the modulator is a set of leaves movable between closed and open states to occlude and pass rays, and wherein for each leaf, a proportion of time in the open state is controlled during intervals of axis angle according to the radiation treatment plan to produce the high flux incomplete tomographic projection set; and wherein the electronic computer executes the stored program to position the proportion of time in the open state after a beginning of each angular interval and wherein the low flux data is collected at the beginning of each angular interval. 
     
     
       6. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the modulator includes multiple leaves having open and closed positions to pass or occlude rays and wherein the low flux rays are obtained by opening of leaves to pass rays not required by the radiation treatment plan. 
     
     
       7. The combination radiation therapy and tomographic imaging machine of  claim 6  wherein the modulator is a set of binary actuated leaves movable between closed and open states to occlude and pass rays, and wherein for each leaf a proportion of time in the open state is controlled during intervals of axis angle according to the radiation treatment plan to produce the high flux incomplete tomographic projection set; and wherein the electronic computer executes the stored program to center the open state within each angular intervals and wherein the low flux data is collected at the center of each angular interval. 
     
     
       8. The combination radiation therapy and tomographic imaging machine of  claim 7  wherein the modulator provides a set of controllable leaves operable for modulating rays, and wherein the electronic computer executes the stored program to optimize a pattern of operating the leaves to satisfy the radiation treatment plan given an opening of the leaves to generate the low flux data. 
     
     
       9. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the radiation source provides a controlled emission of radiation and wherein the low flux rays are obtained by controlling the radiation source. 
     
     
       10. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the electronic computer further executes the stored program to: 
       control the modulator to direct selected low flux rays at selected axis angles at the patient within the treatment volume according to a radiation treatment plan, the selected rays and angles comprising less than a tomographic projection set, to acquire a low flux incomplete tomographic projection set from the radiation detector; and  
       combine the augmented tomographic projections set and the low flux incomplete tomographic projection set.  
     
     
       11. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the electronic computer further operates to: 
       control the modulator to direct selected high flux rays at selected axis angles at the treatment volume without the patient according to a radiation treatment plan, the selected rays and angles comprising less than a tomographic projection set, to acquire a normalizing high flux incomplete tomographic projection set from the radiation detector;  
       direct low flux rays through the treatment volume without the patient to acquire normalizing low flux data from the radiation detector;  
       prior to combining the high flux incomplete tomographic projection set and the low flux data to produce an augmented complete tomographic projection set; normalizing the high flux incomplete tomographic projection set with the normalizing high flux incomplete tomographic projection, and normalizing the low flux data with the normalizing low flux data.  
     
     
       12. The combination radiation therapy and tomographic imaging machine of  claim 11  wherein the data acquired from the radiation detector for reconstruction is obtained only at times when the beam is not being modulated. 
     
     
       13. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the acquisition of the high flux incomplete tomographic projection set is interleaved with acquisition of the low flux data as the axis angle is changed. 
     
     
       14. The combination radiation therapy and tomographic imaging machine of  claim 1  wherein the modulator is a set of actuated leaves movable between closed and open states to occlude and pass rays, and wherein for each leaf a proportion of time in the open state is controlled during intervals of axis angle according to the radiation treatment plan to produce the high flux incomplete tomographic projection set; and wherein the electronic computer executes the stored program to position the open state a predetermined time after a beginning of each angular interval and wherein the low flux data is collected at the beginning of each angular interval. 
     
     
       15. A combination radiation therapy and tomographic imaging machine comprising: 
       a megavoltage radiation source directing a radiation beam formed of rays directed generally along an axis;  
       a gantry holding the radiation source to rotate the angle of the axis about a treatment volume;  
       a radiation detector positioned on the gantry across the treatment volume opposite the radiation source to receive and measure radiation of the rays to provide data of a projection;  
       a modulator modulating the flux of the rays passing through the treatment volume; and  
       an electronic computer executing a stored program to:  
       (i) control the modulator to direct selected high flux rays at selected axis angles at the patient within the treatment volume according to a radiation treatment plan, the selected rays and angles comprising less than a tomographic projection set, to acquire a high flux incomplete tomographic projection set from the radiation detector;  
       (ii) control the modulator to close the leaves to direct low flux rays at the patient within the treatment volume to acquire low flux data from the radiation detector;  
       (iii) combine the high flux incomplete tomographic projection set and the low flux data to produce an augmented complete tomographic projection set; and  
       (iv) reconstruct a tomographic image of the patient from the augmented complete tomographic projection set.  
     
     
       16. The combination radiation therapy and tomographic imaging machine of  claim 15  wherein a proportion of time that each leaf is in the open state is controlled during intervals of axis angle according to the radiation treatment plan to produce the high flux incomplete tomographic projection set; and wherein the electronic computer executes the stored program to position the open state after a beginning of each angular interval and wherein the low flux data is collected at the beginning of each angular interval. 
     
     
       17. A combination radiation therapy and tomographic imaging machine comprising: 
       a megavoltage radiation source directing a radiation beam formed of rays directed generally along an axis;  
       a gantry holding the radiation source to rotate the angle of the axis about a treatment volume;  
       a radiation detector positioned on the gantry across the treatment volume opposite the radiation source to receive and measure radiation of the rays to provide data of a projection;  
       a modulator providing a set of binary actuated leaves movable between closed and open states to occlude and pass rays, and wherein for each leaf a proportion of time in the open state may be controlled during intervals of axis angle;  
       a means for determining windowed times during which the leaves of the modulator are substantially stationary for collecting tomographic data from the radiation detector; and  
       an electronic computer programmed for reconstructing a tomographic image of the patient from the tomographic data collected during the windowed times.